The present disclosure relates generally to optical communications, and more particularly to feed-forward carrier phase recovery.
The popularity of multimedia communications services over packet data networks, such as the Internet, continues to grow; consequently, the demand for higher capacity in core data transport networks continues to grow. For service providers, core data transport networks are optical networks based on fiberoptic technology. To increase the capacity of optical networks, advanced signal modulation techniques, such as quadrature amplitude modulation (QAM) and quadrature phase shift key (QPSK) have been developed. In particular, M-ary QAM (M-QAM) (such as square 16-QAM and 64-QAM) have the potential to realize optical transmission at very high speeds with high spectral efficiency.
Digital coherent detection has proven to be an effective technique for detecting and demodulating the received optical signals. A key step in digital coherent detection is carrier phase recovery. Carrier phase is degraded by laser phase noise in the received optical signal. Laser phase noise is dependent on the linewidth of the optical carrier. For high-order M-QAM modulation formats (M>4), the tolerance for laser phase noise becomes smaller as the modulation level increases, because the Euclidean distance becomes smaller. Consequently, carrier phase recovery methods with improved laser linewidth tolerance are critical for successful implementation of high-order M-QAM modulation formats.
Various carrier phase recovery methods have been developed. One method is based on a decision-directed phase-locked loop. This method has relatively poor laser linewidth tolerance because the phase estimate is based on a previous set of data symbols, not the most current data symbols. In practice, carrier phase recovery methods are implemented in hardware using parallel and pipeline architectures to attain real-time high-speed systems. The tolerance for laser linewidth can then become worse due to extended feedback delay.
A second carrier phase recovery method is based on the classic feed-forward M-th power algorithm using dedicated symbols. Because only a small portion of the symbols can be used for phase estimate for high-order M-QAM, however, this method also has inherently poor laser linewidth tolerance.
A third carrier phase recovery method is based on a blind phase search algorithm. Since this method employs a feed-forward configuration and also involves all the current symbols for phase estimate, it can achieve much better laser linewidth tolerance than the previous two methods. This method, however, is complex because the required number of test phase angles increases with the modulation level. For high-order M-QAM, the required number is very high; for example, >32 is required for square 64-QAM. Since testing a single phase angle by itself requires a series of computationally intensive steps (rotate a set of data symbols, make a decision, and calculate the mean squared error), the computational power required for real-time testing of a large number of phase angles is very high.